Known cellular radio telephone networks such as those conforming to the American Mobile Phone System (AMPS), the Enhanced Total Access Communication System (ETACS) and the Nordic Mobile Telephone System (NMT) specifications, employ Frequency-Division Duplex (FDD) operation. In a system using FDD both the receiver and transmitter of a radio telephone's transceiver are required to be capable of operating simultaneously. Known radio telephone networks operate at a band of frequencies around 900 MHz. The duplex frequency separation between receive and transmit signals is 45 MHz and the total bandwidth for receive and transmit bands is 25 MHz for NMT and AMF'S systems, and 33 MHz for the ETACS system.
An important aspect of the design of a receiver for a radio telephone is the rejection of spurious responses, many of which originate from the non-linear elements comprising the RF stage of the receiver. Since the receiver has to be capable of detecting and processing low power signals, down to -113 dBm (ETACS, NMT) and -116 dBm (AMPS), spurious signals which typically are also low power but higher power than the desired receive signals have a noticeable effect in the receiver. For heterodyne receivers the spectral location, mechanism and rejection of spurious responses is known to the person skilled in the art in transceivers operating in a FDD system as described above, such as the transceiver 100 shown in FIG. 1a, there can be additional spurious responses caused by the presence of a transmit signal from the transceiver during signal reception by the receiver 102. The additional spurious responses are caused by a portion (e.g. -13 dBm) of the high transmit power leaking through the transceiver duplexer 104 into the receiver 102.
One such response is the so-called "half-duplex response". This is generated by signals which lie spectrally mid-way between the receive (Rx) and transmit (Tx) channels (half duplex signals) intermodulating with leaked transmit signals in the receiver's amplifier and mixer to create an interference signal or spurious response at the receive channel frequency. The half-duplex response is caused by third order non-linearities in the front end of RF amplifying and mixing stages of the receiver.
If the receiver is designed to provide half-duplex response rejection, then extra transmit rejection from the receive filters, or a higher third order intercept point (i.e. greater linearity) are required in the non-linear devices or both. This results in larger, more expensive transceivers since more filters and/or tunable filters which take up greater space are required for higher filter rejection requirements, or reduced standby battery time since greater current input power (LO power) or higher voltages are required to give higher amplifier and mixer linearity.